The invention relates to packet radio networks in general, and in particular to supporting mobility in packet radio networks.
General packet radio service GPRS is a new service in the GSM system, and is one of the objects of the standardisation work of the GSM phase 2+ at the ETSI (European Telecommunication Standard Institute). The GPRS operational environment comprises one or more subnetwork service areas, which are interconnected by a GPRS backbone network. A subnetwork comprises a number of packet data service nodes SN, which in this application will be referred to as serving GPRS support nodes SGSN, each of which is connected to the GSM mobile communication network (typically to base station systems) in such a way that it can provide a packet service for mobile data terminals via several base stations, i.e. cells. The intermediate mobile communication network provides packet-switched data transmission between a support node and mobile data terminals. Different subnetworks are in turn connected to an external data network, e.g. to a public switched data network PSPDN, via GPRS gateway support nodes GGSN. The GPRS service thus allows packet data transmission between mobile data terminals and external data networks when the GSM network functions as an access network.
FIG. 1A illustrates a GPRS packet radio network implemented in the GSM system. The basic structure of the GSM system comprises two elements: a base station system BSS and a network subsystem NSS. The BSS and mobile stations MS communicate over radio links. In the base station system BSS each cell is served by a base station BTS. A number of base stations are connected to a base station controller BSC, which controls the radio frequencies and channels used by the BTS. Base station controllers BSC are connected to a mobile services switching centre MSC. As regards a more detailed description of the GSM system, reference is made to the ETSI/GSM recommendations and The GSM System for Mobile Communications, M. Mouly and M. Pautet, Palaiseau, France, 1992, ISBN:2-957190-07-7.
In the system shown in FIG. 1 the GPRS system connected to the GSM network comprises one GPRS network, which in turn comprises two serving GPRS support nodes (SGSN) and one GPRS gateway support node (GGSN). The different support nodes SGSN and GGSN are interconnected by an intra-operator backbone network. In a GPRS network there may be any number of support nodes and gateway support nodes.
The serving GPRS support node SGSN is a node which serves a mobile station MS. Each support node SGSN controls a packet data service within the area of one or more cells in a cellular packet radio network, and therefore each support node SGSN is connected (via a Gb interface) to a certain local element of the GSM system. This connection is typically established to the base station system BSS, i.e. to base station controllers BSC or to a base station BTS. A mobile station MS located in a cell communicates with a base station BTS over a radio interface and further with the support node SGSN to the service area of which the cell belongs through the mobile communication network. In principle, the mobile communication network between the support node SGSN and the mobile station MS only relays packets between these two. To realise this, the mobile communication network provides packet-switched transmission of data packets between the mobile station MS and the serving support node SGSN. It has to be noted that the mobile communication network only provides a physical connection between the mobile station MS and the support node SGSN, and thus its exact function and structure are not significant with respect to the invention. The SGSN is also provided with a signalling interface Gs (e.g. an SS7 signalling connection) to the visitor location register VLR of the mobile communication network and/or to the mobile services switching centre. The SGSN may transmit location information to the MSC/VLR and/or receive requests for paging a GPRS subscriber from the MSC/VLR.
When the MS attaches to the GPRS network, i.e. in a GPRS attach procedure, the SGSN creates a mobility management (MM) context containing, for example, information related to the mobility and security of the MS. In connection with a PDP activation procedure the SGSN creates a PDP (packet data protocol) context which is used for routing purposes within the GPRS network with the GGSN which the GPRS subscriber uses.
The GPRS gateway support node GGSN connects an operator's GPRS network to other operators' GPRS systems and to data networks 11-12, such as an inter-operator backbone network, IP network (Internet) or X.25 network. The GGSN includes GPRS subscribers' PDP addresses and routing information, i.e. SGSN addresses. Routing information is used for tunneling protocol data units PDU from data network 11 to the current switching point of the MS, i.e. to the serving SGSN. Functionalities of the SGSN and GGSN nodes can be integrated into one physical node.
A home location register HLR of the GSM network contains GPRS subscriber data and routing information and it maps the subscriber's IMSI into one or more pairs of the PDP type and PDP address. The HLR also maps each pair of PDP type and PDP address into one or more GGSNs. The SGSN has a Gr interface to the HLR (a direct signalling connection or a connection via an internal backbone network 13). The HLR of a roaming MS may be in a different mobile communication network than the serving SGSN.
An intra-operator backbone network 13, which interconnects the SGSN and GGSN equipment of an operator, can be implemented, for example, by means of a local network, such as an IP network. It should be noted that a GPRS network of an operator can also be implemented without the intra-operator backbone network, e.g. by providing all features in one computer.
An inter-operator backbone network is a network via which different operators' gateway support nodes GGSN can communicate with one another.
FIG. 1B illustrates protocol layers of the signalling level between an MS and an SGSN. In the GPRS system, layered protocol structures, known as a transmission level and a signalling level, have been defined for transmitting user information and signalling. A transmission level has a layered protocol structure providing transmission of user information together with control procedures of data transmission related to it (e.g. flow control, error detection, error correction and error recovery). A signalling level consists of protocols which are used for controlling and supporting the functions of the transmission level, such as controlling access to the GPRS network (Attach and Detach) and controlling the routing path of the established network connection in order to support user mobility. The protocol layers of the transmission level are identical with those of FIG. 2 up to protocol layer SNDCP, above which there is a protocol of the GPRS backbone network (e.g. the Internet Protocol IP) between the MS and the GGSN (instead of protocol L3MM). The protocol layers illustrated in FIG. 1B are:                The Layer 3 Mobility Management (L3MM): this protocol supports the functionality of mobility management, e.g. GPRS Attach, GPRS Detach, security, routing area update, location area update, activation of a PDP context, and deactivation of a PDP context.        The Subnetwork Dependent Convergence Protocol (SNDCP) supports transmission of protocol data units (N-PDU) of a network layer between an MS and an SGSN. The SNDCP layer, for example, manages ciphering and compression of N-PDUs.        The Logical Link Control (LLC): this layer provides a very reliable logical link. The LLC is independent of the radio interface protocols mentioned below.        The LLC Relay: this function relays LLC protocol data units (PDU) between an MS-BSS interface (Urn) and a BSS-SGSN interface (Gb).        The Base Station Subsystem GPRS Protocol (BSSGP): this layer transmits routing information and information related to QoS between a BSS and an SGSS.        The Frame Relay, which is used over the Gb interface. A semipermanent connection for which several subscribers' LLC PDUs are multiplexed is established between the SGSN and the BSS.        The Radio Link Control (RLC): this layer provides a reliable link independent of radio solutions.        The Medium Access Control (MAC): this one controls access signalling (request and grant) related to a radio channel and mapping of LLC frames onto a physical GSM channel.        
With respect to the invention, the most interesting protocol layers are the LCC and the L3MM. The function of the LLC layer can be described as follows: the LLC layer functions above the RLC layer in the reference architecture and establishes a logical link between the MS and its serving SGSN. With respect to the function of the LCC the most important requirements are a reliable management of the LCC frame relay and support for point-to-point and point-to-multipoint addressing.
A service access point (SAP) of the logical link layer is a point where the LLC layer provides services for the layer 3 protocols (the SNDCP layer in FIG. 1B). The link of the LLC layer is identified with a data link connection identifier (DLCI), which is transmitted in the address field of each LLC frame. The DLCI consists of two elements: A Service Access Point Identifier (SAPI) and a Temporary Logical Link Identity TLLI. When a more general expression of a TLLI is needed, the term ‘temporary identity’ will be used.
When a user attaches to a GPRS network, a logical link is established between the MS and the SGSN. Thus it can be said that the MS has a call in progress. This logical link has a route between the MS and the SGSN, indicated with the TLLI identifier. Thus the TLLI is a temporary identifier, which the SGSN allocates for a certain logical link and IMSI. The SGSN sends the TLLI to the MS in connection with the establishment of a logical link, and it is used as an identifier in later signalling and data transmission over this logical link.
Data transmission over a logical link is carried out as explained in the following. Data to be transmitted to or from an MS is processed with an SNDCP function and transmitted to the LLC layer. The LLC layer inserts the data in the information field of LLC frames. The address field of a frame includes e.g. a TLLI. The LLC layer relays the data to the RLC, which deletes unnecessary information and segments the data into a form compatible with the MAC. The MAC layer activates radio resource processes in order to obtain a radio traffic path for transmission. A corresponding MAC unit on the other side of the radio traffic path receives the data and relays it upwards to the LLC layer. Finally, the data is transmitted from the LLC layer to the SNDCP, where the user data is restored completely and relayed to the next protocol layer.
Three different MM states of the MS are typical of the mobility management (MM) of a GPRS subscriber: an idle state, a standby state and a ready state. Each state represents a certain functionality and information level, which has been allocated to the MS and the SGSN. Information sets related to these states, called MM contexts, are stored in the SGSN and the MS. The context of the SGSN contains subscriber data, such as the subscriber's IMSI, TLLI and location and routing information, etc.
In the standby and ready states the MS is attached to the GPRS network. In the GPRS network, a dynamic MM context has been created for the MS, and a logical link LLC (Logical Link Control) is established between the MS and the SGSN in a protocol layer. The ready state is the actual data transmission state in which the MS can transmit and receive user data. The MS switches from the standby state to the ready state either when the GPRS network pages the MS or when the MS initiates data transmission or signalling. The MS may remain in the ready state (for a period set with a timer) even when no user data is transmitted nor signalling performed.
In the standby and ready states the MS also has one or more PDP contexts (Packet Data Protocol), which are stored in the serving SGSN in connection with the MM context. The PDP context defines different data transmission parameters, such as the PDP type (e.g. X.25 or IP), PDP address (e.g. X.121 address), quality of service QoS and NSAPI. The MS activates the PDU context with a specific message, Activate PDP Context Request, in which it gives information on the TLLI, PDP type, PDP address, required QoS and NSAPI. When the MS roams to the area of a new SGSN, the new SGSN requests MM and PDP contexts from the old SGSN.
For mobility management, logical routing areas have been defined for the GPRS network. A routing area (RA) is an area defined by an operator, comprising one or more cells. Usually, one SGSN serves several routing areas. A routing area is used for determining the location of the MS in the standby state. If the location of the MS is not known in terms of a specific cell, signalling is started with a GPRS page within one routing area RA. In other words, a paging area is normally also a routing area in a GPRS system, and a location area in a current GSM system.
The MS performs a routing area update procedure in order to support mobility of a packet-switched logical link. In the ready state the MS initiates the procedure when a new cell is selected, the routing area changes or an update timer of a cyclic routing area expires. The radio network (PLMN) is arranged to transmit a sufficient amount of system information to the MS so that it can detect when it enters a new cell or a new routing area RA and to determine when it is to carry out cyclic routing area updates. The MS detects that it has entered a new cell by comparing cyclically the cell identity (Cell ID) which is stored in its MM context with the cell identity which is received from the network. Correspondingly, the MS detects that it has entered a new routing area RA by comparing the routing area identifier stored in its MM context with the routing area identifier received from the network. When the MS selects a new cell, it stores the cell identity and routing area in its MM context.
All the procedures described above (e.g. attach, detach, routing area update and activation/deactivation of the PDP context) for creating and updating MM and PDP contexts and establishing a logical link are procedures activated by the MS. In connection with a routing area update the MS, however, carries out an update to the new routing area without being able to conclude on the basis of the routing area information broadcast by cells whether the SGSN serving the new cell is the same as the SGSN that served the old cell. On the basis of the old routing area information transmitted by the MS in an update message the new SGSN detects that a routing area update is in progress between two SGSN nodes, and it activates the necessary interrogation to the old SGSN in order to create new MM and PDP contexts for the MS to the new SGSN. Since the SGSN has changed, the logical link should be re-established between the MS and the new SGSN.
FIG. 2, which is originally FIG. 17 of ETSI Recommendation GSM 03.60 (version 6.0.0), is a signalling diagram illustrating (mainly) a prior art attach procedure. The mobile station's former support node SGSN and mobile switching centre MSC/VLR are called “old” and the current ones are called “new”. In step 2-1 the MS sends an ATTACH REQUEST. Steps 2-2 to 2-5 are not necessary for understanding the invention and these steps will not be described. In step 2-6a the new node, SGSN2, sends an UPDATE LOCATION message to the HLR, which in step 2-6b sends a CANCEL LOCATION to the old SGSN1. In step 2-6c the old SGSN1 acknowledges (=ACK). In step 2-6d the new SGSN2 receives the subscriber's data in a message INSERT SUBSCRIBER DATA and acknowledges in step 2-6e. In step 2-6f the new SGSN2 receives from the HLR an acknowledgement to the location update sent in step 2-6a.
In step 2-7a the new SGSN2 sends a LOCATION UPDATING REQUEST to the new MSC/VLR. Steps 2-7b through 2-7g correspond to steps 2-6a through 2-6f. In step 2-7h the new SGSN2 receives from the new MSC an acknowledgement to the location update sent in step 2-7a. In step 2-8 the new SGSN2 reports to the MS that the ATTACH REQUEST sent in step 2-1 has been accepted. The remaining steps are not relevant to the invention and will not be described.
FIG. 3, which is originally FIG. 26 of ETSI Recommendation GSM 03.60 (version 6.0.0), is a signalling diagram illustrating (mainly) a prior art routing area update procedure. In an inter-SGSN routing area update procedure the serving SGSN changes and the MS should be informed of the change so that the MS can initiate a local procedure or a network procedure for updating a logical link. In the following description, the reference numbers refer to messages or events shown in FIG. 3.
3-1. The MS sends a routing area update request to the new SGSN2. This message includes the temporary logical link identity TLLI, the cell identity of the new cell Cell_id, the routing area identifier of the old routing area RA_id, and the routing area identifier of the new routing area RA_id. If load is to be decreased in the radio interface, the cell identity Cell_id is not added until in the base station system BSS.
3-2. The new SGSN2 detects that the old routing area belongs to another SGSN, which will be referred to as an old node, SGSN1. As a result, the new SGSN2 requests MM and PDP contexts for the MS in question from the old SGSN1. All contexts can be requested at the same time, or the MM context and each PDP context can be requested in different messages. The request(s) includes at least the routing area identifier RA_id of the old routing area and the TLLI. The old SGSN1 sends in response an MM context, PDP contexts and possibly authorization parameter triplets. If the MS is not recognized in the old SGSN1, the old SGSN1 replies with an appropriate error message. The old SGSN1 stores the new SGSN2 address until the old MM context has been deleted so that data packets can be relayed from the old SGSN1 to the new SGSN2.
3-3. The new SGSN2 sends a message “Modify PDP Context Request” including e.g. a new SGSN address to the GGSNs concerned. The GGSNs update their PDP context fields and send in response a message “Modify PDP Context Response”.
3-4. The new SGSN2 informs the HLR of the change of the SGSN by sending a message “Update Location” including a new SGSN address and an IMSI.
3-5. The HLR deletes the MM context from the old SGSN1 by sending it a message “Cancel Location” including an IMS1. The old SGSN1 deletes the MM and PDP contexts and acknowledges this by sending a message “Cancel Location Ack”.
3-6. The HLR sends a message “Insert Subscriber Data” including an IMSI and GPRS subscriber data to the new SGSN2. The new SGSN2 acknowledges this by sending a message “Insert Subscriber Data Ack”.
3-7. The HLR acknowledges the location update by sending a message “Update Location Ack” to the SGSN.
3-8. If the subscriber is also a GSM subscriber (IMSI-Attached), the association between the SGSN and the VLR has to be updated. The VLR address is deduced from the RA information. The new SGSN transmits a message “Location Updating Request” including e.g. an SGSN address and an IMSI to the VLR. The VLR stores the SGSN address and acknowledges by sending a message “Location Updating Accept”.
3-9. The new SGSN2 confirms the presence of the MS in the new routing area RA. If there are no restrictions for registration of the MS for the new RA, the SGSN creates MM and PDP contexts for the MS. A logical link will be established between the new SGSN and the MS. The new SGSN2 replies to the MS with a message “Routing Area Update Accept” including e.g. a new TLLI. This message tells the MS that the network has succeeded in carrying out the update.
3-10. The MS acknowledges the new TLLI with a message “Routing Area Update Complete”.
The above-described procedures for allocating the TLLI identifiers, performing routing/location area updates and paging the mobile station are based on several years of experience with GSM systems, and they have been found satisfactory. However, these procedures rely on the assumption that the identifier of the SGSN nodes can be derived from the identities of the cells they serve. It is conceivable that in the future this assumption may no longer be valid. For example, one paging area could be handled by several network elements, such as SGSN nodes. Alternatively, one network element could serve many paging areas. This scenario presents two problems, namely:
When the mobile station changes its paging area, the new supporting network element may have trouble in determining the old supporting network element on the basis of the paging area identifier. There is also a risk of two supporting network elements allocating the same TLLI to two different mobile stations.